As is well known, four-wire and two-wire communications paths are commonly coupled by a hybrid circuit. Due to imperfect balancing of the hybrid circuit, a component of a signal incoming on the four-wire receive path, referred to as an echo signal, is inevitably but undesirably coupled to the four-wire transmit path, with an attenuation which is referred to as the echo return loss.
In order to cancel the echo signal, it is well known to provide an echo canceller which comprises an adaptive filter and a subtraction unit. The adaptive filter is supplied with the signal incoming on the four-wire receive path and produces an estimated echo signal, which the subtraction unit subtracts from the signal on the four-wire transmit path to produce an outgoing four-wire transmit path signal which includes a residual echo signal. The adaptive filter is adapted in dependence upon the residual echo signal in a manner which seeks to reduce the residual echo signal to zero, i.e. in such a manner that the estimated echo signal corresponds exactly to the actual echo signal coupled via the hybrid circuit. The echo return loss enhancement, or ERLE, is the degree to which the echo canceller suppresses the echo signal, i.e. the ratio of the echo signal to the residual echo signal.
As is also well known, it is necessary to inhibit adaptation of the adaptive filter whenever there is a so-called near end signal present, such a signal being coupled from the two-wire path to the four-wire transmit path, because such a signal constitutes noise as far as the convergence of the echo canceller is concerned. It is therefore common to provide a so-called double-talk detector which detects simultaneously-occurring signals in both directions of transmission, adaptation of the adaptive filter being inhibited in response to such detection.
A double-talk detector typically monitors the average signal levels on the four-wire receive path and on the four-wire transmit path prior to the subtraction unit, and determines a double-talk condition, i.e. that a near end signal is present, if the latter average signal level exceeds the former signal level reduced by the ERL. However, the ERL is unknown and must be estimated. Generally a fixed value, typically 6 dB, is arbitrarily selected and assumed for the ERL.
In practice, the ERL may change during a communication due to changes in line impedance or signal routing, and will change from one call to the next for networked echo cancellers. The actual ERL can range from 0 dB to 18 dB, so that the fixed value generally used can be a poor estimate of the actual ERL.
In the event that the estimated value of the ERL is less than the actual value, a double-talk condition can exist without being detected. In this case adaptation of the adaptive filter, i.e. updating of filter coefficients, will continue and the echo canceller will diverge from its desired state. Conversely, if the estimated value of the ERL is greater than the actual value, the echo signal can be mistaken for double-talk so that the adaptive filter will not be adapted and the echo canceller will never converge to its desired state.
Various references disclose the use of a fixed value for the ERL in a double-talk detector as discussed above. In addition, Rohrs et al. U.S. Pat. No. 4,918,727 issued Apr. 17, 1990 and entitled "Double Talk Detector For Echo Canceller And Method" describes a double-talk detector which monitors the ERLE to detect double-talk. This patent recognizes that an end path switch may result in a false double-talk detection and consequent failure to adapt the adaptive filter, and provides an end path switch detector to complement the ERLE monitoring for more reliable double-talk detection. However, obtaining a good estimate of actual ERL is not addressed in this prior art.
An object of this invention, therefore, is to facilitate determining echo return loss in an echo cancelling arrangement, so that a good estimate of the actual echo return loss can be provided. This good estimate can be used for example by a double-talk detector to provide a relatively accurate detection of a double-talk condition for inhibiting adaptation of the adaptive filter of the echo canceller.